sys: introduce generic driver

This driver is intended to complete system-specific drivers that don't
have implemented all required driver functionality. Currently, it
implements offset functions working on top of system-specific frequency
functions. Offsets are corrected by changing frequency, similarly to
fast slewing implemented in the Linux driver.

logging.h

@@ -81,6 +81,7 @@ typedef enum {
   LOGF_Rtc,
   LOGF_Regress,
   LOGF_Sys,
+  LOGF_SysGeneric,
   LOGF_SysLinux,
   LOGF_SysNetBSD,
   LOGF_SysSolaris,